Hybrid energy conversion and processing detector

ABSTRACT

A hybrid arrangement of more than one electron energy conversion mechanism in an electron detector is arranged such that an image can be acquired from both energy converters so that selected high-illumination parts of the electron beam can be imaged with an indirectly coupled scintillator detector and the remainder of the image acquired with the highsensitivity/direct electron portion of the detector without readjustments in the beam position or mechanical positioning of the detector parts. Further, a mechanism is described to allow dynamically switchable or simultaneous linear and counted signal processing from each pixel on the detector so that high-illumination areas can be acquired linearly without severe dose rate limitation of counting and lowillumination regions can be acquired with counting.

CROSS REFERENCE TO RELATED APPLICATIONS

This non-provisional utility application claims priority as a divisionalapplication of non-provisional application Ser. No. 14/688,081 filedApr. 16, 2015 and to provisional application No. 61/981,138, filed Apr.17, 2014, both applications being entitled “High Energy Conversion andProcessing Detector.” The entire disclosures of both applications areincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates generally to the field of electron microscopyimage detection.

BACKGROUND OF THE INVENTION

For much of the history of electron microscopy up until the 1990s, sheetfilm was the main image recording medium. In 1990, electronic detectorsbased on scintillators coupled to scientific semiconductor image sensorswith either fused fiber or lens optics were introduced and began toreplace film for most image recording applications. One of the mainadvantages of these sensors was for the applications of electron energyloss spectroscopy (EELS) and electron diffraction (ED) due thedramatically increased dynamic range of thesescintillator-optics-semiconductor sensor. Both of these applicationsinvolve the use of a focused beam, with specimen information derivedfrom electrons which are diverted from the main path either due toenergy loss (subsequently made manifest with a bending magnet) or due toelectron diffraction. The deflected beams can be several to many ordersof magnitude weaker than the un-deflected beam as shown in FIGS. 1A, 1B,2A and 2B. The graph in FIG. 1B shows that the diffraction spots in FIG.1A have dynamic range of several orders of magnitude. FIGS. 2A and 2Bshow the dynamic range needed in EELS. Semiconductor sensors, typicallyCCDs for imaging and photodiode arrays and CCDs for spectroscopy, havesignificantly higher dynamic range than film, especially scientificsensors which were made with larger pixels. Dynamic ranges of up to20,000 have been shown possible in single exposures with merged multipleexposures extending that limit even further. In addition, scintillatorscoupled optically to the sensor protect the sensor from the radiationdamage effects that can results from either direct exposure of thesensor to the beam or from exposure to x-rays generated at thescintillator which can then travel to the sensor. In the case of lenscoupling, the glass of the lens and the distance the lens allows to becreated between scintillator and sensor confer the protective effect. Inthe case of fused fiber optic plates, the high density glass providesthe x-ray protection. In both cases, the beam is stopped long before itcan hit the sensor and cause damage directly. For both these reasons,the scintillator/optic/semiconductor sensor has replaced film in 100% ofdiffraction applications and made possible the parallel acquisition ofspectra, which wasn't an option at all before these sensors made itpossible.

FIG. 3A shows a prior art lens-coupled scintillator indirect detectorhaving a scintillator 301, a glass prism 302, optical lenses 303, 304and a CCD detector 305. FIG. 3B shows a prior art fused fiber-opticplate coupled indirect detector having a scintillator 311, an opticalfiber bundle, 312 and a CCD detector 313.

Certain imaging applications, most notably cryo-electron microscopy ofproteins and cellular cryo-tomography drove a need for a replacement offilm which did not have a dynamic range requirement but rather asensitivity and resolution requirement. In the last few years, a classof detectors has been developed using radiation hardened silicon activepixel detectors which is now successfully replacing film and extendingthe resolution limits attainable in structural biology. This technologyis usually referred to as direct detection technology and cameras usingthis technology as direct detectors. In contradistinction to thesedetectors, the scintillator/optic/sensor detectors described above arenow commonly referred to as indirect detectors.

Direct detectors make these improvements in structural biologyresolution in a number of ways. First, silicon, being lighter thantypical scintillator and optical materials, scatters the electron beamless giving a finer point spread function. Second, the directly detectedincoming electron makes a much stronger signal. Third, it is possible tothin the device to allow the beam to pass through without additionalnoisy backscatter. While thinning is possible with lens coupledscintillators (see, for example, U.S. Pat. No. 5,517,033 all referencescited herein are incorporated by reference), thinning comes with asevere loss of signal strength. FIG. 3C shows a prior art non-thinned(“bulk”) direct detector 320 and FIG. 3D shows a prior art back-thinneddirect detector 330. In a direct detector there is no sensitivitypenalty for thinning.

Finally, as a result of the first three benefits described above,(improved point spread function, higher signal strength and reducedbackscatter from thinning), the signal from a direct detector can beprocessed to result in a counting mode analogous to that often used witha photomultiplier tube and with the same benefit. When an incomingelectron is counted as a 1and added into a frame buffer, the variationsin energy deposited by the incoming electron are stripped off and notsummed as they would be in the case for a linear, integrating detector,whether direct or indirect. Because the acquired image no longercontains that variation in energy, a nearly noise-free acquisition ispossible.

A second variant of counting is possible in which the position of entryof the electron is estimated to sub-pixel accuracy by centroiding thedeposited energy. This method is commonly referred to asuper-resolution. FIG. 4 shows the detection of a single incidentelectron with a super-resolution detector. FIG. 4A shows an electronlanding on an arbitrary location within a pixel on a multipixeldetection device. FIG. 4B shows scatter from incoming electrons in alocalized region near the point of entry to the electron. FIG. 4c showshow the amount of scattered detected signal in nearby pixels is relatedto the location of the electron's entry point. FIG. 4d shows thatselection of the pixel with the highest scattered signal locates thepoint of entry to the nearest pixel. FIG. 4e shows that finding thecenter of mass of the distribution of scattered charge allows locationof the entry point to sub-pixel accuracy.

Counting and super-resolution dramatically improve the sensitivityperformance as measured by detective quantum efficiency (DQE), the ratioof detected signal to noise ratio to incoming signal to noise ratio overthe performance of a silicon direct detection imager.

FIG. 5 shows the sensitivity improvement summarized: of direct detectionover indirect detection (A), of counting direct detection over justdirect detection (B) and of the effect that super-resolution adds signalover the Nyquist frequency (C) of the physical pixel. Part of the DQEbenefit comes from the additional benefit of counting of dramaticallyreducing detection of background noise. It is clear that indirectdetectors have a serious disadvantage in terms of sensitivity. This isboth in terms of DQE as shown in the graph in FIG. 5, but also in termsof the background noise. Both Electron energy loss spectroscopy (EELS)and electron diffraction (ED) have a strong need for high sensitivityand good background rejection for the weak parts of the signal.

Direct detectors (DD) have the drawback that the electron, whiledeposing signal energy in the pixel, will also damage it due to chargeinjected into insulators, as well as knock-on damage to the siliconcrystal structure. This gives direct detectors a lifetime dose limit.While this dose limit has been dramatically increased by improvements topixel design layout, reductions in feature sizes which reduce oxidethickness and thereby allow trapped electrons to diffuse out morereadily, and thinning, which eliminates the energy deposited bybackscatter, total lifetime dose is still limited to significantly lowerlevels than that of fiber-optically or lens-optically coupledscintillators. This fact would be severely limiting in applications likeEELS and ED for which the undeflected beam is often many orders ofmagnitude higher than the low intensity part of the signal and wouldreduce the sensor lifetime, which is measured in years for cyro-electronmicroscopy, to hours.

For a DD detector to count electron arrival events they must bespatially and temporally separated on the detector. DQE is only modestlyaffected for 300 kV electrons at event densities up to about 0.025 perpixel (˜40 pixels per electron event). This is accomplished by anextreme speedup in frame-rate. A framerate of 400 fps as used on theGatan K2 Counting direct detection camera allows a dose rate of 10electrons per pixel per second at that event density. While that doserate is adequate for low-dose imaging in cryo-microscopy for which itwas developed, it is too low for use in higher-dose and in high-dynamicrange applications such as EELS and ED as described in FIGS. 1 and 2.While it is conceivable that frame rates could be increased enough tohandle medium dose ranges as shown in FIG. 2B, it is unrealistic tothink that counting could be used for the high intensity parts of eitherthe EELS or the ED signals. For the current generation of K2 countingdirect detector, a commercial camera developed by Gatan, Inc. which usesthe prior art detector arrangement of FIG. 3D in conjunction withextremely fast readout to separate the incoming electron events intodifferent frames and a fast processor to count or centroid the electronevents and sum them, the useable dose rate is 400 times lower incounting mode than in linear mode. FIG. 6 shows the sparsification byspeed needed to allow counting. On the left, the actual signal generatedby a sparse beam is shown, illustrating that each event covers multiplepixels, with varying size. Sparsification needs to be sufficient toprevent miscounting or poor centroiding due to overlap of scatter fromone event onto another. The image on the right of FIG. 6 shows theresults of counting the frame on the left. It also illustrates theextent to which the variability both in event intensity and in eventsize is reduced through the counting process.

Relevant patents in the field include U.S. Pat. No. 7,952,073(“Bilhorn”) and U.S. Pat. No. 8,334,512 (“Luecken”). All referencescited herein are incorporated by reference in their entirety.

The inadequacy of indirect detectors in resolution and sensitivity fordealing with the weak parts of EELS and ED signals and the inadequacy ofdirect detection to deal with the strong parts of EELS and ED signalscoupled with the lack of any workaround in the prior art for acquiringboth strong and weak signals simultaneous with high quality creates aneed for a new solution.

Electron energy loss spectroscopy (EELS) and electron diffraction (ED)would stand to benefit significantly if the capability to read theweakest signals were added to existing capability. This is especiallytrue for scanning transmission electron microscope spectrum imaging(STEM SI), for which a spectrum is taken at each of a N×M raster ofscanned specimen pixels and used to derive elemental and electroniccontrast images. See Gatan datasheet, “GIF Quantum”, published March2014. This technique, which requires high speed to cover a reasonablenumber of pixels over a reasonable area of a specimen requires highsensitivity to very weak signals in the regions used for elementalcontrast and yet still needs to be able to acquire and digitize theun-deflected beam for normalization. Similar applications are beingdeveloped for electron diffraction with STEM with similar requirements.

U.S. Pat. No. 8,334,512 is a patent in the field and discusses use of afast detector positioned below the thinned imaging detector at somedistance as a zero-loss beam position detector but due to the poorresolution associated with its position, cannot be used as a detectorfor low-loss spectroscopic information.

In addition, EELS and ED signals in their most general application can,and especially in these STEM applications, do vary rapidly in time,making serial illumination first of a low-sensitivity, high-robustnessdetector and second of a high-sensitivity low-robustness detector animpractical solution for the high dynamic range application.

Therefore there exists a need in the prior art for a technology whichwould allow simultaneous robust and high-quality imaging ofhigh-intensity and weak signals in the same field of view.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is an image relating to the prior art;

FIG. 1B is a graph of intensity for the image in FIG. 1B;

FIG. 2A is a graph relating to the prior art;

FIG. 2B is a graph relating to the prior art;

FIG. 3A is a prior art indirect detector coupling design;

FIG. 3B is a prior art indirect detector coupling design;

FIG. 3C is a prior art direct detector design;

FIG. 3D is a prior art direct detector design;

FIG. 4A is a diagram of electron detection;

FIG. 4B is a diagram of electron detection

FIG. 4C is a diagram of electron detection

FIG. 4D is a diagram showing electron detection event signal processing;

FIG. 4E is a diagram showing electron detection event signal processing;

FIG. 5 is a graph describing use of detective quantum efficiency (DQE)as a measure of sensitivity;

FIG. 6 are two images showing the effect of high speed readout toseparate events into different frames (left) and the effect of countingthose events (right);

FIG. 7A shows a hybrid detector design using a fiber-optically coupledscintillator on part of the monolithic detector array;

FIG. 7B shows a hybrid detector design using a fiber-optically coupledscintillator on part of the monolithic thinned detector array;

FIG. 7C shows a hybrid detector design using a fiber-optically coupledscintillator on part of the monolithic back-illuminated thinned detectorarray;

FIG. 7D shows a hybrid detector design using a lens-coupled scintillatoron part of the monolithic front- (or back-) illuminated thinned detectorarray;

FIG. 7E shows a hybrid detector design using a fiber-optically coupledscintillator on part of the monolithic thinned detector array withcoordinated simultaneous readout of both optically-coupled and directdetection portions of the detector;

FIG. 7F shows a hybrid detector design using a fiber-optically coupledscintillator on part of the monolithic thinned detector array withindependent simultaneous readout of both optically-coupled and directdetection portions of the detector;

FIG. 8A shows a dual detector having a scintillator in contact with andon top of a direct detector with the scintillator being opticallycoupled to a second detector;

FIG. 8B shows a dual detector having a scintillator in contact with andbelow a direct back-illuminated detector with the scintillator beingcoupled by a skewed fiber bundle to a second detector;

FIG. 8C shows a dual detector having a scintillator in contact with andbelow a direct thinned back-illuminated detector with the scintillatorbeing coupled by a mirror and optical lenses to a second detector;

FIG. 9A shows a dual detector having a scintillator not in contact withand in front of a direct detector, lens coupled to a separate opticalsensor with means for synchronizing the two detectors;

FIG. 9B shows a dual detector having a scintillator-coupled indirectdetector next to and in close proximity to a direct back-illuminateddetector with the scintillator being coupled by a skewed fiber bundle toenable the very close juxtaposition of the electron detection planes ofthe two detectors, with means for synchronizing the two detectors;

FIG. 10A shows an exemplary hybrid energy conversion detector with threetypes of data read out simultaneously from three separate regions of thedetector; and

FIG. 10B shows a further exemplary hybrid energy conversion detectorwith three types of data read out simultaneously from three separateregions of the detector in such a way that both the counted and thelinearly read direction detection data is merged to allow optimalintegration of both types of data.

DETAILED DESCRIPTION OF THE INVENTION

In an embodiment of the invention, there is disclosed a hybridarrangement of more than one electron energy conversion mechanism in adetector arranged physically such that the electron image can beacquired from both energy converters in such a manner that selectedhigh-illumination parts of the image can be imaged with an indirectlycoupled scintillator detector and the remainder of the image acquiredwith the high-sensitivity/direct electron portion of the detectorwithout readjustments in the beam position or mechanical positioning ofthe detector parts.

Further, a mechanism is included in the signal processor to allowdynamic switching between counted and linear readout modes so thathigh-illumination areas can be acquired linearly without the severe doserate limitation of counting and low-illumination regions can be acquiredwith counting to provide the very high signal quality needed for lowdose and long exposures. Alternatively or in addition, the ability toperform simultaneous linear and counted signal processing from eachpixel of the image is provided to allow subsequent selection orcombination of linear and counted signals offline after acquisition.Both methods would make use of a switchover illumination intensity abovewhich the transition from counted to linear would be made. Theswitchover would be the illumination intensity at which the signalquality was the same for counted and linear modes. Signal quality wouldtypically be measured using detective quantum efficiency (DQE), as usedin FIG. 5 to compare different detectors. DQE of a counting detectorbegins to degrade as illumination rate grows above the level of 1electron per 40 pixels and reaches the level of a linear detector at adose rate between 1 electron per 20 pixels and 1 electron per 10 pixels.In this manner, each pixel in the image can be read out and processed inthe most optimal fashion.

Further, because counted and linear modes have different transfer curves(the functional relationship of illumination to counts) and frequencyresponse or modulation transfer function (MTF), signal processing willbe provided to remove the intensity and resolution differences betweenthe linear and counted signals.

Hybrid detector realizations can be categorized by location ofscintillator (in contact with the detector or not, above direct detectoror below) by coupling means (fused fiber optic plate or lens-coupling),by which side of the direct detection device is coupled to (front sideor back side), by whether the direct detection device is thinned or not,by readout means (single-sensor, all read out together, single sensorwith integrated split readout, and dual sensor configurations), byprocessing means (linear, counted or both). Many combinations of thesefactors can be envisioned. A number of representative combinations areshown in FIGS. 7-10. FIG. 7A-7D show various options for coupling ascintillator to the direct detection device with the scintillator not incontact with the direct detection device. FIG. 7A shows a scintillator701 above the device coupled by a fiber optic plate 702 to the frontside 703 of a non-thinned bulk silicon direct detection device 704. FIG.7B show the same arrangement as FIG. 7A but coupled to the front side705 of a thinned direct detection device 706. FIG. 7C shows ascintillator 701 fiber-optically coupled 702 to the backside 707 of aback-thinned direct detection device 706. And FIG. 7D shows ascintillator 702 lens-optically coupled 708 to a frontside thinneddirect detection device. FIGS. 7E and 7F illustrate two possibilitiesfor readout of the direct detection device 706 which could apply to anycoupling type or position but are here shown in conjunction with thesame fiber-optically coupled scintillator arrangement of FIG. 7B. FIG.7E shows the detector 706 being read out via a unified and coordinatedmechanism 710 that reads out the whole of the device, optically coupledand direct-detection in the same way. FIG. 7F shows a device with thereadout split in to two sections 711, 712 at the location of thetransition from optical coupling to direct detection. This arrangementwould allow for the high-intensity low-loss and zero loss beams to beread out independently and potentially faster than the direct detectionportion of the detector. Faster readout would then attenuate the signalstrength in each readout of the high-intensity signal and allow agreater dynamic range as a result.

FIGS. 8A through 8C show example configurations which place thescintillator 801 in contact with the direct detection device 802. Inthese configurations it is necessary to couple the light to a seconddetector 804 which must be placed out of the way of the incident beam.This is because while light generated in the scintillator can bedetected by the direct detector the signal will be overwhelmed by thescattered electron beam which will still be detected by the directdetection device under the scintillator. FIG. 8A shows a possiblelens-optical 805 arrangement including a shield 803 to prevent scatteredelectrons from reaching the optical detector 804. A synchronizer 806controls readout of the directly exposed detector 803 and the opticaldetector 804. FIG. 8B shows a fiber-optically coupled arrangement withthe scintillator 801 beneath the detector 802. The scintillator 801 iscoupled to the second detector 804 by a skewed fiber optic plate 807FIG. 8C shows a lens-optically 805 coupled arrangement with thescintillator 804 beneath the detector 802 and using a mirror 812 toredirect the scintillator image to the second detector. A means ofsynchronization is provided to allow the outputs of the two detectors tobe merged into a single hybrid image. The embodiments shown in FIGS.7A-F and 8A-C are only representative and do not constitute all possiblearrangements using this concept. An example of an extension not shownwould be to move the in-contact scintillator to the center of the devicefor the application of diffraction (FIGS. 1A and 1B) using lens-couplingto avoid occluding any part of the electron image or the creation ofbackscatter if located under the device.

FIGS. 9A and 9B show arrangements of two detectors arranged withoutphysical contact of either the scintillator 901 or the detector 904.FIG. 9A shows a lens-coupled camera 902 located in front of the directdetection sensor 904 and with optical design 903, 905 so as to minimizedead pixels between optically-coupled 902 and direct-detection sensor904 s. This design would also serve to minimize scatter of electronsfrom the scintillator onto the direct-detection portion of the hybriddetector. FIG. 9B shows a possible arrangement of two detectors 902, 904with detection surfaces in the same plane. The embodiment in Figure Bincludes a detector 904 designed to minimize dead area, and a shield907. The scintillator 901 is coupled to the light sensor 902 by a skewedfiber optic bundle. As for the previous cases shown in FIGS. 8A-8C, FIG.9A shows a synchronizer 906 to control readout of the directly exposeddetector 904 and the optical detector comprised of scintillator 901,image sensor 902 and optics 905 and 903.

FIGS. 10A and 10B concern image processing options and the extension ofthe hybrid concept to cover a combination of both linearly detected andcounted data. FIG. 10A shows linear indirect, linear direct and counteddirect-detection data being read out by the detector 1001, from thehigh, medium and low-dose portions of the image or spectrum. Theembodiment shown here includes a scintillator 1002 coupled to thedetector by a fiber optic bundle 1003. The position of transition fromlinear direct detection to counted direct detection would be determinedby the illumination pattern and the switchover illumination intensityand would therefore vary from one image to the next—and might happenmore than once in an image. FIG. 10B shows an arrangement that processesevery pixel in the direct detection area both as a linear pixel and withcounting or centroiding. While this increases the amount of data whichneeds to be saved, it creates the possibility of selecting or mergingthe data offline for a more highly optimized synthesis of data. Itshould be clear that the above combinations are just representative of amuch larger set of combinations.

With respect to FIG. 8, Separate detectors could be operated with orwithout synchronization. Exposure times could be the same or differentfor the indirect and direct portions of the detector. With thescintillator in direct contact with the direct detector, the directdetector will be exposed to the intense beam and so will age quickly.However, since that part of the detector is not needed due to thepresence of the indirectly coupled sensor, it can be allowed to becomenon-functional.

With respect to FIG. 9, Separate detectors could be operated with orwithout synchronization. Exposure times could be the same or differentfor the indirect and direct portions of the detector.

The invention claimed is:
 1. A hybrid processing directly illuminatedtwo-dimensional detector (HPDD) comprising: a two-dimensional imagesensor comprising a plurality of pixels, and a processor configured to:simultaneously perform linear acquisition of a first portion of saidplurality of pixels and event-counting acquisition of a second portionof said plurality of pixels, wherein when performing the event-countingacquisition of the second portion of said plurality of pixels, theprocessor is configured to find a center of mass of a distribution ofscattered charge with respect to event energy associated with a firstpixel in said second portion of pixels and with pixels adjacent to saidfirst pixel.
 2. The HPDD detector of claim 1, wherein the processor isfurther configured to simultaneously linearly acquire and event countacquire every pixel of the two dimensional image sensor.
 3. The HPDDdetector of claim 1, wherein the processor is further configured toadjust signal levels of said first portion of pixels and said secondportion of pixels to minimize image merge artifacts.
 4. A hybridprocessing directly illuminated two dimensional electron detector (HPDD)comprising: a two-dimensional image sensor comprising a plurality ofpixels, and a processor configured to: linearly acquire a first portionof said pixels, and event-count acquire a second portion of said pixels,wherein said event-count acquisition of said second portion of saidpixels includes a super-resolution computation based on finding a centerof mass of a distribution of scattered charge with respect to eventenergy associated with a first pixel in said second portion of pixelsand with pixels adjacent to said first pixel.
 5. The HPDD of claim 4,wherein which pixels are acquired linear and which pixels are acquiredby event counting are dynamically configurable.
 6. The HPDD of claim 5,wherein said dynamic configuration of which pixels are acquired linearlyand which pixels are acquired by event counting is based on an overalldose rate received by said two-dimensional image sensor.
 7. The HPDD ofclaim 4, wherein said processor is further configured to adjust signallevels of said linearly acquired pixels and said event-counted acquiredpixels to minimize image merge artifacts.
 8. A hybrid energy-conversiondetector (HECD) for receiving a beam of electron radiation havingpredetermined high and low illumination intensity portions, the detectorcomprising: an energy converting scintillator exposed to the highillumination intensity portion of the beam of electron radiation, saidscintillator generating light from said high illumination intensityportion of the beam of electron radiation; a two-dimensional imagesensor comprising a plurality of pixels that includes: a first imagesensor portion arranged and adapted for receiving said light from saidenergy converting scintillator and for processing a first image portion;and a second image sensor portion arranged and adapted for directlyreceiving the low illumination intensity portion of the beam of electronradiation and for processing a second image portion, wherein saidscintillator is exposed to the high illumination intensity portion ofthe beam of electron radiation, wherein said second sensor portion isexposed to said low intensity portion of the beam of electron radiation,wherein said exposure of said first image sensor portion to said lightand said exposure of said second sensor portion to the low illuminationintensity portion of the beam of electron radiation occurs at least inpart simultaneously; and a processor configured to perform a linearacquisition of a first portion of pixels in said second image sensorportion and an event-counting acquisition of a second portion of pixelsin said second image sensor portion, wherein said event-countingacquisition includes a super-resolution computation including finding acenter of mass of a distribution of scattered charge with respect toevent energy associated with a first pixel in said second portion ofpixels and with pixels adjacent to said first pixel.
 9. The HECD ofclaim 8, wherein said processor is further configured to allowsimultaneous linear acquisition and event counting acquisition of everypixel of said second image sensor portion.
 10. The HECD pixel array ofclaim 8, wherein said processor is further configured to adjust signallevels of said linearly acquired pixels and said event-count acquiredpixels to minimize image merge artifacts.
 11. The HECD of claim 8,wherein which pixels are acquired linearly and which pixels are acquiredby event counting is dynamically configurable.
 12. The HECD of claim 11,wherein said dynamic configuration of which pixels are acquired linearlyand which pixels are acquired by event counting is based on overall doserate to said second image sensor portion.
 13. The HECD pixel array ofclaim 8, wherein said processor is further configured to adjust signallevels of said linearly acquired pixels and said event-count acquiredpixels to minimize image merge artifacts.